The capacity of the vertebrate nervous system depends critically on the formation of appropriate neural connections. Functional neuromuscular connections are established during embryogenesis, when axons navigate with high precision to their muscle targets. This process of axonal pathfinding depends on properly specified neurons expressing a particular complement of guidance receptors, as well as on properly specified cells which interact with migrating growth cones and provide guidance cues. The objective of the studies described here is to discover genes that govern migration of spinal motor axons to their muscle targets. Embryonic spinal motor neurons are anatomically well described, easy to identify and their axonal paths have been defined in much detail, providing an excellent model system in which to study how spinal neurons establish precise and stereotyped neural connections with the body musculature. Previous studies have shown that a specialized group of somitic cells, the adaxial cells, play a pivotal role in the migration of motor axons from the spinal cord to their synaptic muscle targets. Thus, a genetic screen specifically for defects in motor axons and adaxial cells will provide the basis for a molecular-genetic analysis of how motor neurons establish their neural connections with the body musculature. The experiments in this proposal will: (1) mutagenize zebrafish and screen an equivalent of 1,800 mutagenized genomes in third generation embryos for defects in spinal motorneuron and adaxial cell development (through labeling with motor axon and adaxial specific antibodies); (2) define the primary defect in the isolated mutants (through molecular marker analysis for cell survival, specification, differentiation and migration of motor neurons and adaxial cells); and (3) as a first step towards their molecular identity, determine the genetic map position of the mutated genes (through molecular genetic mapping). The study will identify multiple genes, including genes essential for cell survival, specification, development and migration of spinal motor neuron and adaxial cells. The studies will provide a foundation to address the mechanisms underlying human hereditary diseases, including neuropathies and muscular atrophies.